The electrophysiology of dopamine (DA)-containing neurons of the midbrain has been extensively studied. This interest derives, in part, from the association of DA systems with several neurological and psychiatric disorders. Since some of these disorders are age-related, it is of importance to determine if changes in the functioning of DA neuronal pathways occur with age. Biochemical and behavioral indices of the status of midbrain DA pathways do, in fact, show that a decline in function occurs during aging. Despite this knowledge, no study to date has explored the effects of aging on the electrophysiology of midbrain DA-containing neurons. The proposed studies have been designed to begin to increase our understanding of the effects of aging on the electrical activity of A9 and A10 DA cells. Single unit in vivo electrophysiological recording techniques will be employed to accomplish this goal. In particular, the spontaneous activity of A9 and A10 DA neuronal populations in rats between the ages of 3 and 28 months will be assessed. Thus, age-related changes in the number of spontaneously active DA cells will be determined as will any changes in the firing rate and pattern of the A9 and A10 DA neuronal populations. In addition, the effect of aging on the sensitivity of identified (by antidromic activation from terminal region) nigrostriatal, mesolimbic and mesoprefrontal DA neurons to the inhibitory effects of DA agonists will be assessed. It is hoped that this initial investigation of the effects of aging on DA neuronal electrophysiology will lead to a better understanding of age-related alterations in the physiology and pharmacology of midbrain DA neurons. This knowledge, combined with that obtained with other techniques, may prove helpful in advancing our hypotheses regarding the effects of aging on DA neuronal function.